1. Field of the Invention
The present invention relates to an electroplating device useful for electroplating a work having a hole communicating with the outside, particularly, a ring-shaped work such as a ring-shaped bonded magnet, and a process for electroplating such a work using the device.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet, of which an Nd—Fe—B based permanent magnet is representative, is used at present in a variety of fields, because it is produced from an inexpensive material rich in natural resources and has a high magnetic characteristic.
In recent years, in electronic and appliance industries where a rare earth metal-based permanent magnet is used, a reduction in size of each of parts has been advanced, and in correspondence to this, it is necessary to reduce the size of the magnet itself and to form the magnet into a complicated shape.
From this viewpoint, public attention is paid to a bonded magnet which is easy to form into a certain shape from a material containing a magnetic powder and a resin binder as main components. Among others, a ring-shaped bonded magnet is utilized, particularly, in various small-sized motors such as a spindle motor, or in a servomotor used in an actuator.
The rare earth metal-based permanent magnet contains a rare earth metal (R) which is liable to be corroded by oxidation in the atmosphere. Therefore, when the magnet is used without being subjected to any surface treatment, the corrosion of the magnet is advanced from the surface due to the presence of a small amount of an acid, an alkali or moisture to produce a rust, and as a result, the deterioration and variability of the magnetic characteristic of the magnet occur. Therefore, a plated film has been conventionally formed as a corrosion-resistant film on a surface of a magnet by subjecting the magnet to an electroplating, but a higher accuracy is required in the formation of the plated film, attendant on the recent demands for the reduction in size of the magnet and for the complication of the shape.
In the case of the ring-shaped bonded magnet, the high dimensional accuracy is required for both of the outer and inner surfaces of the magnet and hence, a uniform plated film must be formed on the outer surface, but also a uniform plated film must be formed particularly on the inner surface. In the case of a ring-shaped bonded magnet having a large L/D value (wherein L represents a length of the magnet in a direction of a center axis, and D represents an inside diameter of the magnet), the following problem is encountered: An area near a central portion of the inner portion of the magnet is lower in current density, resulting in a plated film formed at a smaller thickness. In addition, if air bubbles produced upon the immersion of the ring-shaped bonded magnet into a plating bath and hydrogen gas produced during the electroplating are resident on an inner upper portion of the magnet, they exert a deleterious influence to the formation of a plated film on such portion.
To subject a recessed portion provided in a work to an electroplating, it is a conventional practice that an anode is inserted into and disposed in such portion (for example, see Japanese Patent Application Laid-open No.3-6399). However, when the anode is merely inserted and disposed, the distance between the inner surface of the magnet and the anode cannot be averagely regularized. Therefore, an obtained effect is only that a plated film can be formed efficiently on the inner surface, and the variability of formation of the plated film from portion to portion of the inner surface cannot be overcome.
In addition, if the distance between the outer surface of the magnet and a positive electrode plate is averagely not regularized, the variability of formation of a plated film from portion to portion of the outer surface cannot be overcome.
Further, in electroplating processes proposed hitherto, traces of contact with a plating electric current supplying member and a work fixing member are left on a work and for this reason, a post-treatment is required, which impedes the formation of a uniform plated film.